2fAcids, alkalis and titrations

The three indicators

Phenolphthalein is colourless in acid — not white, not clear. Colourless is the word examiners expect.

What a single indicator can and cannot tell you

Each of these changes colour once, so it answers one question: acid or alkali?

It cannot separate acidic from neutral. Phenolphthalein is colourless in both, so a colourless result narrows things down without settling them. To identify an acid you need universal indicator or a pH meter.

Methyl orange has the same limitation at the other end: it is yellow in neutral solutions as well as alkaline ones.

The pH scale

The specification classifies solutions like this:

Learn these five bands — they are examined directly.

One piece of wider truth worth knowing: pH describes the solution, not the acid itself. A very dilute strong acid can reach pH 5 just as a concentrated weak acid can. The bands describe how acidic the solution is, which depends on concentration as well as on the acid.

Neutral means exactly 7. pH 6 is weakly acidic, not "about neutral".

Universal indicator

Universal indicator is a mixture of several indicators that change at different pH values. That is why it produces a continuous range of colours rather than a single switch, and why it can give an approximate pH.

To use it: add a few drops (or dip the paper), then compare the colour with a pH chart. The comparison is where the number comes from — an answer stopping at "it turns red" has not measured anything.

What makes something an acid or an alkali

The phrase in aqueous solution is part of the definition. A pure acid with no water present does not behave as an acid at all — the water is what allows the ions to form and move.

Neutralisation

When the two meet, the ions combine:

H⁺ + OH⁻ → H₂O

That ionic equation is the same for every acid and alkali pair, which makes it worth memorising once rather than deriving each time.

Both ions are removed from solution, which is why the pH moves towards 7.

Watching it happen

Add sodium hydroxide drop by drop to hydrochloric acid containing universal indicator, and the colour tracks the pH:

• Red at the start — strongly acidic.
• Green at the point of neutralisation — pH 7.
• Purple once excess alkali has been added.

Green is neutral for universal indicator. All three colours mean something, and a full answer accounts for the last one too — purple shows you have gone past the end point.

When the end point is exactly pH 7

A strong acid neutralised by a strong alkali gives a neutral solution at pH 7.

Other combinations do not land exactly there, because the salt formed is itself slightly acidic or alkaline. On this specification you will usually be given a strong acid and a strong alkali, but it is worth knowing why the qualification exists.

Neutralisation outside the laboratory

Farmers add calcium hydroxide to acidic soil. The alkali neutralises the acid, raising the pH towards 7, which suits most crops better.

Note the direction: adding an alkali to an acid raises the pH. Saying it falls is a straightforward error and an easy one to make under time pressure.

Separate Chemistry only.

A titration finds exactly how much of one solution neutralises another. Its accuracy comes from using the right apparatus for each job.

The apparatus, and why each

The pipette fixes one quantity exactly; the burette measures however much is needed to reach the end point. Swapping them loses the accuracy of both.

The method

1. Pipette 25.0 cm³ of acid into a conical flask.
2. Add a few drops of a suitable indicator.
3. Fill the burette with alkali and record the starting reading.
4. Add alkali, swirling the flask, until the indicator changes colour permanently.
5. Add dropwise near the end point.
6. Record the final reading and calculate the titre.
7. Repeat until concordant results are obtained.

Which indicator

Not universal indicator. Its gradual colour range gives no sharp end point — the very property that makes it good for measuring pH makes it useless here.

Phenolphthalein or methyl orange are used, because each changes colour over a narrow pH range and gives a sharp end point for a strong acid with a strong alkali.

Concordant results

Concordant means the titres agree within 0.10 cm³ of each other.

The first titration is usually a rough one, done quickly to find the approximate end point. It is excluded from the mean.

Given 24.10, 23.60, 23.65 and 23.55 cm³, average the last three. Including the rough 24.10 would raise the mean and give a wrong concentration.

Repeating without checking the results agree does nothing for reliability — the point is identifying and discarding anomalies.

Rinsing

Apparatus is rinsed with the solution it will contain, not with water. Water left in a burette dilutes the alkali, so a greater volume is needed to neutralise the acid, and the titre comes out too large.